In a population-based sample, COPD was associated with a greater risk of depressive symptoms compared to a matched referent group. The fact that increasing COPD severity is associated with an increasing likelihood of depressive symptoms provides further evidence for an association between COPD and depression. Depressive symptoms also appeared to negatively impact quality of life, highlighting the importance of depression in these patients. Targeting depression in COPD could therefore be an attractive strategy to improve health status.
Previous research on the risk of depression faced by patients with COPD has yielded conflicting results.5–13
The study by van Manen and colleagues,11
with 162 COPD subjects, is the largest prior study on this subject that also included both a referent population and pulmonary function measurement. Interestingly, this study showed an increase in depressive symptoms among subjects with severe COPD (FEV1
<50% of predicted) but not in their cohort as a whole or in the less severe subgroups.11
We cannot be sure of the reason for our differing conclusions, but this may have to do with higher rates of mood disorders generally in the United States relative to The Netherlands, where that study was performed.33
Prior research has suggested that cultural differences between nations may affect the expression of depressive symptoms and be responsible for differing risk factors for major depression in various countries.34
Our finding that higher COPD severity is associated with a higher likelihood of depressive symptoms provides further validity to the concept that COPD is associated with depression. However, our cross-sectional analysis cannot completely determine the causal pathway between COPD and depressive symptoms. It is possible that COPD causes depression or that depression causes COPD via its association with cigarette smoking.35
Both pathways may also be operative. Alternatively, shared genetic and environmental factors may predispose independently to both smoking and depression.36, 37
Thus, because COPD may not be directory causing depression, targeting improved respiratory physiology in COPD may not alleviate depressive symptoms.
We found that depressive symptoms were strongly associated with health-related quality of life. Even though the risk of depressive symptoms increased with increasing COPD severity, the relationship between depressive symptoms and health-related quality of life was present taking into account such severity. These results are particularly important because depression complicating COPD is often overlooked in clinical practice.37
Our results suggest that attempts to improve the quality of life of COPD patients should not underestimate the importance of depression as a potentially mediating factor. Moreover, there is evidence that treating depression in COPD improves quality of life.38, 39
Although this might include antidepressive pharmacotherapy,39
interventions such as pulmonary rehabilitation, which often includes psychosocial support, may also improve mood and reduce depressive symptoms.4, 40–42
Alternatively, psychological counseling within the context of a physician visit may be important.43
Because of the strong association between depressive symptoms and quality of life, further study regarding effective methods of treating depression in COPD appear clearly warranted.
Several study limitations must be considered. Although the inclusion criteria required healthcare utilization for COPD and COPD medication usage, it is possible that some subjects had asthma rather than COPD. However, all patients also had a physician diagnosis of COPD and reported having the condition. The observed lifetime smoking prevalence was similar to that in other population-based epidemiologic studies of COPD, supporting the diagnosis of COPD rather than asthma.44, 45
Nonetheless, we cannot exclude the possibility that some subjects, especially those with less obstruction on spirometry, may have conditions other than COPD. However, we note that reduced FEV1
was associated with a higher likelihood of depressive symptoms; thus, eliminating subjects with higher FEV1
from our cohort would only have strengthened our finding that subjects with COPD are at higher risk of depressive symptoms than referents.
Because an important focus in the prospective follow-up of our cohort will be studying the long-term prevention of COPD-associated morbidities, we intentionally sampled younger adults with COPD (ages 40–65). Although we adjusted for age within our multivariate models, we cannot be sure of the applicability of our results to older adults with COPD. In addition, KP members, because they have health care access, may also be different than the general population of adults with COPD. Mitigating this limitation, the sociodemographic characteristics of Northern California KP members are similar to those of the regional population. 46, 47
Moreover, selection bias could have been introduced by non-participation in the study and could in turn affect the generalizability of our results. However, our participation rates were comparable or better than many other studies on this subject.1, 6, 7, 9–13
Our measure of depression, the GDS, is not intended to diagnose major depression but rather depressive symptoms. Nonetheless, patients with depressive symptoms warrant further evaluation in clinical practice, and treatment for depression is often recommended even in the absence of a diagnosis of major depression.48
One advantage of the GDS over other measures of depressive symptoms is that it is less contaminated with somatic symptoms such as poor appetite and poor sleep that may be symptoms of either depression or of COPD itself. 9, 11
This reduces the likelihood that we are over-estimating depression in this setting.
In conclusion, we found that patients with COPD, at all levels of airway obstruction, were at higher risk of depressive symptoms than referents. Furthermore, increasing COPD severity was associated with an increasing likelihood of depressive symptoms. Even so, after taking COPD severity into account, depressive symptoms were strongly associated with worse quality of life.